This invention relates to a prefilter both in the form of a kit for retrofit as well as part of the original construction for an electrical discharge machining (EDM) machine. EDM is a process in which a conductive electrode is charged with electricity and brought into contact with a workpiece to thereby electrically erode away the unwanted portion thereof. Typically, a thin wire (from 0.002 inches to 0.014 inches diameter) is used as the electrode and is thus capable of precision machining. EDM is a very precise and complicated machining process that is typically controlled by an on-board computer.
All wire EDM machines come standard with a cartridge-type filter system for filtering the water used to flush the swarf or waste from the workpiece as it is burned away. Proper flushing of the workpiece is critical in attaining the high degree of accuracy and reliability required for most EDM operations. The standard cartridge filter on most wire EDM machines can be relatively expensive and, under heavy machining conditions, require replacement on a relatively frequent basis. This not only becomes expensive due to the cost for the replacement filter elements, but also from machine down time as it can take from one to two hours to change out a filter element for a typical EDM machine. Several examples of primary filter systems as might be used on an EDM machine include U.S. Pat. Nos. 3,757,949; 2,664,203; and 2,665,812; the disclosures of which are incorporated herein by reference.
In order to minimize the expense and down time normally associated with the primary filter systems for EDM machines in the prior art, the inventors herein have succeeded in developing a prefilter which can be utilized as a kit to retrofit existing EDM machines and which can be incorporated into the design of new EDM machines. Essentially, the prefilter of the present invention includes a tank with a plurality of holes along its bottom, with a mounting bracket for holding a roll of continuous, sheet-type filter paper, channels on opposite sides of the bottom of the tank through which the filter paper is threaded to a take-up roll mounted opposite the filter paper roll, and a float switch in the tank for sensing the level of water in the tank and actuating a motor operatively coupled to the take-up roll for advancing the filter paper through the tank. The tank is designed to fit over the top of the water tank of an EDM machine and under the return water line such that the flush water which contains machine residue fills the prefilter tank and is processed therethrough prior to being filtered by the primary filter of the EDM machine.
The prefilter design of the present invention is elegantly simple and makes efficient use of relatively inexpensive continuous sheet-type filter paper in order to prefilter the swarf from the flush water. In operation, as the flush water fills the tank, it filters through the filter paper and exits the tank through the series of holes along the tank bottom. As the filter paper becomes clogged with swarf, the water flow rate decreases such that the level of the water in the tank increases. Once the water level reaches a pre-determined point, the float switch operates which actuates the motor on the take-up roll to advance the filter paper along the bottom of the tank. This increases water flow through the filter paper as dirty filter paper is advanced out of the bottom of the tank while clean filter paper is advanced into the bottom of the tank, clean filter paper obviously having a greater flow rate than dirty filter paper. As the water flow rate through the filter paper increases, the tank level decreases and the float switch turns off the motor to stop further movement of the filter paper through the tank. This process is repeated as the new filter paper becomes clogged as well. The inventors have found that in a typical installation, the float switch remains on for approximately three seconds and the filter paper moves an average of one inch for each cycle. The float switch may cycle as much as once every five minutes to an hour, depending upon the duty cycle for the EDM machine.
While the principal advantages and features of the invention have been explained above, a more detailed explanation follows in the drawings and description of the preferred embodiment.